Noise reduction system



June 4, 1940- l v. D. LANDON I 2,203,455

` NOISE REDUCTION SYSTEM.

Filed May 28, 1938 3 Sheets-Shet 1 kmkkwlb .n .www

.l Sworn EUILLTdNH Sn mmol' Cttorncg June 4, 1940. .v. D. LANDON H0152 REDUCTION SYSTEM Filed may 2a', 1958 3 Sheets-Sheetl 2 `:inventent s shets-sheet 5 Vernon D. Landon v 4 dttorneg v. D. LANDON NoIsE REDUCTION SYSTEM Flled May 28, 1958 June 4, 1940.

30:-broadly tuned stages.

Patented June 4, 1940 "Pf-WENT4 OFFicE 2,203,465 s NOISE REDUCTION sYs'rEM Vernon D. Landon, naaaonnela, N. J., assigner to Radio Corporation of Americana corporation of Delaware Application May 28, 1938,: Serial No.`210,581 claims. (c1. estL-2o) This invention relates to noise reduction systems such as are utilized to eliminate undesired electrical impulses from the output of radio receiver apparatus and the like, and has for its s principal object the provision of an improved system and method of operation whereby the broadcast signals reaching the limiting element of the system are largely restricted to those originating at the station from which reception is lil* desired. I

In connection with systems of this character, it is well known that the amplifier` stages preceding the limiter should be broadly tuned and that `the ampliiier stages following` the `limiter l5 t shouldbe sharply; tuned if the noise is to be most effectively reduced. In this type of system, however, there is encountered the difficulty that unwanted broadcast signals from adjacent stations in the broadcast band are likely to saturate the `z-limiter and prevent its proper functioning to `separate the desired signal from the noise im` pulses superimposed on it. In accordance with the invention, this difficulty is avoided by the provision of a series of wave traps which are in- 25. terposed between the antenna and `the first amplifier stage and are so ganged or mechanically coupled with the tuning-elements of the sharply tuned stages that each unwanted broadcast signal is prevented from reaching Vthe input of the This allows the broadly tuned stages ahead of the limiter to cover the full width of the broadcast band except for the signal ranges excluded by the wave traps.

The invention will be better understood Afrom the following description considered in connection with the accompanying drawings and its scope is indicated by the appended claims. s

Referring to the drawings- Figure 1 is a block diagram illustrating cero tain features of the invention,

Figures 2 and 2A are wiring diagrams of the system of Fig. 1, and

Figures 3 and 4 are explanatory curves relating tothe operation of the invention.

The system of Fig. 1 includes an antenna Ill from which signal impulses are supplied through wave traps II, I2 and I3, a broadly tuned ampli- I jected for the most part only to broadcast signals originating at the station to which the sharp stages `are tuned.

As appears from Figs. 2 and 2A, which are taken together to show the complete signal channel, `the power for operating the various stages is derived from a bleeder resistor I8 provided with a high potential lead I9 and a plurality of intermediate potential leads 2li, 2l and 22.

The kbroadly tuned section of the'channel in` cludes twostages 23 and 24.` The stage 23 has its input l circuit vcoupled to the antenna lil through `a transformer 25 and its output circuit coupled to the input circuit of the stage 24 through a transformer 26. `The output circuit of the stage 2li is coupled through a transformer `21 to the control grid circuit of a limiter 28. As is customary, the stage 23 is `provided with the usual condenser shunted cathode lead bias resistor `and* with a reslstor connected in shunt to its output winding for ensuring broad tuning.

The stage 34 is similarly provided with a cathode lead resistor 3l and an output coil shunt resistor 32.

, It is also desirable that the broadly tuned section of the channel be provided with an automatic volume control circuit for keeping the peal; voltage of allthe continuous wave voltages added together to such a value as to barely fill the limiter. `How such a circuit is applied to the broadly tuned `section `of the channel is so well known as to render its illustration unnecessary.

It should be noted that screen grid voltage is applied to the broadlytuned amplifiers 23 and 24 and the tubes 34, 36 and 3l through the intermediate potential lead 23, and that the anode and screen grid voltages of the limiter 28 are derived respectively through the leads 22 and 2|. Under these conditions, the amplitude ofthe electrical impulses passing through the limiter 28 is restricted to a value predetermined by saturation of the limiter which is operated at a relatively low cathode-anode voltage.

ooupledrto the output circuit of the limiter 2s through a resistance coupling including the capacitor 33 is a relatively low impedance stage 34 which has its output circuit coupled through a transformer 35 (transfer from Fig. 2 to Fig. 2A) to the input of sharply tuned stages 36 and 3l. The output circuit of the sharply tuned amplifier 31 is coupled through a transformer 38 and 42 and that these tuning capacitors are ganged or mechanically coupled through a unitary control member, indicated by the broken line 43, to a switch member 44 arranged to engage one after the other of the contacts 45-46, I1- 48 and 49-50 of the Wave traps I I, I2 and I3. As previously indicated, the switch member v44 and the tuning capacitors 4D, 4I and 42 are so correlated mechanically that the wave trap tuned to the desired station is rendered inoperative. f

The remainder of the signal channel requires little explanation. It includes an audio frequency stage 5I which derives its control grid potential from the diode output resistor 52 through a coupling capacitor 53 and a second audio frequency stage 54 provided with an input circuit coupled to the output circuit of the stage 5I through a transformer 55 and with an output circuit coupled to the loudspeaker through a transformer 5B.

As indicated by Fig. 3, a decoy train due to a static pulse is of short duration in a broad amplier. The sharp wave trap puts a long "tail on the wave train. Due to the fact that this tail Vis of a frequency to which the trap is tuned, it is not heard in the receiver which is tuned to a different frequency. The tail (see Fig. 4) has a long time duration but a low amplitude and hence does not actuate the limiter. The high amplitude and short time duration portion of the wave train is limited by saturation of the limiter 28 or in any other suitable manner and produces little or no effect on the output afterpassing through the `sharp amplifier stagesl and 54. y i

I claim as my invention:

1. In a `noise reduction system wherein a limiter is interposed between a broadly tuned signal section and a sharply tuned signal section followed by a detector, the combination of a plurality of wave traps connected ahead of said broadly tuned section for selectively controlling the frequency of the broadcast signals transmittedv to said limiter, and unicontrol means for maintaining a predetermined relation between the selection of said wave traps and the tuning of said sharply tuned section.

2. In a noise reduction system wherein a limiter is interposed between a broadly tuned signal section and a vsharply tuned signal section follower by a detector, the combination of a plu'- rality of wave traps connected ahead of said broadly tuned section for selectively controlling the frequency of the broadcast signals transmitted to said limiter, and unicontrol means for maintaining a predetermined relation between the selection of said wave traps and the tuning of said sharply tuned section and said detector.

3. Ina noise reduction system, a signal amplifying channel comprising, in combination, a broadly tuned signal section followed by a sharply tuned signal section, signal. limiter means interposed between said sections, means for tuning said second named section to a predetermined signal frequency, and means external of said broadly tuned section for minimizing in said broadly tuned section signals at frequencies other 'than said predetermined signal frequency.

4. In a noise reduction system, a signal amplifying channel comprising, in combination, a broadly tuned signal section followed by a sharp- 1y tunedy signal section, `signal limiter means interposed between said sections, means for tuning said second named section to a predetermined signal, means `for minimizing in said broadly tuned section signals other than said predetermined signal, and means for simultaneously and conjointly actuating said last named means and said means for tuning said second named section in predetermined relation to each other. 1

5. In a noise reduction system wherein a limiter is interposed between ,a broadly tuned signal section and a sharply tunedv signal section followed by a detector whereby received signals pass through said broadly tuned signal section to the limiter and thence through the sharply tuned signal section to the detector in the order named, the combination of a plural` ity of wave traps connected ahead of said broadly tuned section forfselectively controlling the frequency of the broacast signals transmitted to said limiter, means for Variably tuning said sharply tuned signal sectionthrough a predetermined signal selecting range, and unicontrol means for maintaining a predetermined relation 4between the selection of said wave traps and the tuning of said sharply tuned section and said detector by said last named means.

. VERNON D. LANDON. 

